What does Hebrews 7:10 mean?

The author of Hebrews is attempting to show that the figure of Melchizedek (Genesis 14:14–24) is greater than even Abraham. This will fit into an upcoming argument that the priesthood order of Melchizedek is superior to that of the Levitical priests, which explains why Jesus Christ is the true, perfect means of our salvation. Verses 9 and 10 do this by hinting at a concept known as federal headship, where a people's ancestors are spoken of as their "head," or "leader," as if those who were born later were part of their ancestor. A classic example is that of Adam, used by Paul in the book of Romans: Adam sinned, and we were all "in" Adam at that time (Romans 5:12).

Here, the point is being made with a direct statement that it's a figure of speech. The prior verse used the Greek phrase hōs epos eipein, literally meaning "so to speak." Levi, like all of the priests who descended from him, was figuratively "still in the loins" of Abraham when Abraham honored Melchizedek with a tithe. In that sense, even the Levitical priests can be said to have offered a tithe to a greater figure. This summarizes the author's argument about Abraham and Melchizedek, setting up his next point about the priesthood held by Jesus Christ.